FIG. 1 illustrates a modern, twin engine jet aircraft of known design and FIG. 2 the engine installation for such an aircraft. The aircraft comprises fuselage 10, wings 12 and 14, and essentially identical engine installations 16 and 18 supported by wings 12 and 14 respectively. Engine installation 16 comprises nacelle 20 within which a high bypass turbofan engine is mounted. Such an engine includes a turbine casing housing a high-pressure compressor, a burner, and a turbine. The turbine is connected to a central shaft running along the centerline of the engine, and the central shaft in turn drives the high-pressure compressor. A large diameter, high bypass annular fan is mounted near the forward end of nacelle 20, and is driven from the turbine shaft. The engine utilizes some of the air passing through inlet 22 to burn jet aircraft fuel in the turbine engine, to produce primary exhaust plume 24 of hot gases from primary exhaust nozzle 26. The fan expels the remainder of the air introduced via inlet 22 through fan discharge duct 28 as fan flow 30. Typical temperatures for primary exhaust plume 24 are 700.degree.-800.degree. F. Typical temperatures for fan flow 30 are 90.degree.-120.degree. F. Both primary exhaust plume 24 and fan flow 30 have generally cylindrical shapes about centerline 25.
The diameter of nacelle 20 of a high bypass, tubofan engine is significantly larger than the diameter of the nacelle of a low bypass tubofan engine or of the turbine casing of a turbo jet engine. FIG. 2 illustrates the comparatively small clearance 21 between ground line 23 and the lower surface of nacelle 20. To provide adequate ground clearance, it is necessary to position the nacelles of engine installations 16 and 18 closely under the respective wings. Nacelle 20 and the jet engine contained within the nacelle are supported from wing 12 by strut 32. The rearward portions of strut 32 are enclosed by aft fairing panels 34 and 36, and aft wedge panel 38, it being understood that similar aft fairing panels 34 and 36 are also disposed on the opposite side (as viewed in FIG. 2) of strut 32. The lower edges of aft fairing panels 34 and 36, and the aft fairing panels on the opposite side of strut 32, terminate in lower pan assembly 40 that extends rearwardly from the vicinity of primary exhaust nozzle 26 along the lower edges of the aft fairing panels. Aft fairing panel 34 therefore extends in a generally vertical direction between wing 12 and lower pan assembly 40. Flap track fairing 42 is secured to wing 12 directly behind strut 32, and encloses actuators for the flaps and other control devices on wing 12. Aft fairing panel 36 extends downward from flap track fairing 42 to lower pan assembly 40. Lower pan assembly 40 is divided lengthwise at joints 44-47 into five sections, to permit thermal growth of the lower pan assembly without buckling or deformation.
Further details of aft fairing panel 36, lower pan assembly 40 and related structures are shown in FIG. 3. Aft fairing panels 36 are joined to flap track fairing 42 by steel skate angles 50. In the embodiment illustrated in FIG. 3, each aft fairing panel is fabricated from steel. To reduce weight, the center portions of the steel fairing panels are chemically milled, to produce panels that have comparatively thin center sections and comparatively thick upper end portions 52 and lower end portions 54. Upper end portions 52 are used to join the aft fairing panels to skate angles 50. The lower ends of the aft fairing panels are joined at lower end portions 54 to web 56 that extends between the aft fairing panels. Secured between each aft fairing panel and web 56 is a splice strap 58 extends beneath the web, the splice straps being used for mounting lower pan 60, lower pan 60 being one segment of lower pan assembly 40 of FIG. 2.
In the initial embodiment of the engine installation shown in FIGS. 1-3, skate angles 50 were fabricated from steel, fairing panels 36 were fabricated from aluminum honeycomb, web 56 was fabricated from steel, and lower pan assembly 40 was fabricated from a nickel-steel alloy such as Inconel 625 alloy that has a very low coefficient of thermal expansion. However, it was soon discovered that such an installation resulted in severe thermal effects in aft fairing panels 36, flap track fairing 42 and related structures. The cause of such effects was believed to be the expansion of primary exhaust plume 24 during engine cutback. In particular, when the engine within nacelle 20 is throttled back, such as during descent, fan flow 30 is significantly reduced, and as a result, primary exhaust plume 24 expands and washes over the lower portions of the aft fairing panels and, in particular, after fairing panel 36. The result of such flow was the production of thermal stresses that produced buckling of aft fairing panels 36, and cracking of pan assembly 40, splice straps 58 and skate angles 50. The initial response to this problem was to change aft fairing panels 36 from aluminum honeycomb construction to the steel construction illustrated in FIG. 3. However, it was soon discovered that this solution delayed manifestation of the cracking problems, but did not ultimately prevent cracking and buckling. The only obvious solution was to provide steel internal supports within the area enclosed by the aft fairing panels, in an effort to prevent the buckling and cracking. However, it was recognized that this would be an extremely heavy and expensive solution to the problem.